MDR1 (multiple drug resistance) is a versatile ATP-dependent pump which transports a variety of unrelated substances from the cytoplasm to reduce their intracellular accumulation. MDR1 is expressed in many normal tissues including the luminal surface of intestinal epithelia where my data suggest that mdr1 functions as part of a barrier to xenobiotics. I have found there is significant interindividual variation in the expression of mdr1 mRNA and protein in the small bowel. Furthermore, I have shown this variability accounts for a significant portion of the interpatient differences in oral pharmacokinetics of the mdr1 substrate, cyclosporin A. I believe this variation in mdr1 expression is likely to affect the oral clearance of mdr1 substrates and may represent a previously unrecognized risk factor for environmental diseases. The first hypothesis I will test is that variation in intestinal mdr1 expression is largely due to non-genetic factors. This hypothesis is based on my preliminary data showing mdr1 is inducible in vitro and in vivo in human intestine by rifampin. In Specific Aim 1, I will identify inducers of mdr1 mRNA and protein in small bowel and colon by treating cultured human intestinal explants with suspected inducers. In Specific Aim 2, the inducers identified in Aim 1 will be tested in vivo by measuring mdr1 expression in small and large bowel biopsies obtained endoscopically from healthy subjects after a 4 day inducer free diet and again after short term treatment with an inducer. Intestinal explants from each subject will be simultaneously cultured with the inducer for correlation of in vivo and in vitro results. I will also test the effects of diet on intestinal mdr1 expression by placing healthy volunteers on diets containing charcoal broiled beef or, in collaboration with Paul Watkins, grapefruit juice (flavonoids). The level of mdr1 mRNA and protein will be determined in small and large bowel biopsies obtained at study entry, after a 4 day inducer free diet, and after 7 days on the test diet. Specific Aim 3 will directly test the hypothesis that the level of intestinal mdr1 influences the absorption of drugs in the intestine. I will first examine if the level of mdr1 expression affects the threshold and rate of "absorption" of the mdr1 substrates, cyclosporin A and vinblastine, across cultured Caco-2 cell monolayers. Second, I will determine whether interindividual variation in intestinal mdr1 expression correlates with the variable oral pharmacokinetics of tacrolimus, midazolam, and ethinyl estradiol (collaboration with Paul Watkins and Alan Leichtman). I am confident that the data from these studies will lead me to novel hypotheses regarding the effects of mdr1 expression on xenobiotic absorption and the risk of environmental diseases.